Rocket assembly ablative materials formed from, as a precursor, staple cellulosic fibers, and method of insulating or thermally protecting a rocket assembly with the same

ABSTRACT

A rocket motor assembly is insulated or thermally protected with a rocket motor ablative material formed from a prepreg. The prepreg contains at least an impregnating resin matrix and, as a precursor prior to carbonization, carded and spun staple cellulosic fibers. When patterned and carbonized, the rocket motor ablative material can be lined or otherwise placed into a rocket motor assembly, such as between the solid propellant and case, in the bulk area of the exit nozzle liner, or at susceptible portions of a re-entry vehicle, such as the nose cone.

RELATED U.S. APPLICATIONS

Priority is based on U.S. Provisional Application No. 60/124,674 filedon Mar. 16, 1999 and U.S. Provisional Application No. 60/097,117 filedon Aug. 19, 1998, the complete disclosures of which are incorporatedherein by reference.

ORIGIN OF THE INVENTION

Certain aspects of this invention were made under contract NAS 8-38100with the National Aeronautics and Space Administration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to rocket motor ablative materials, especiallyresin-filled carbon fiber and carbon/carbon ablative materials, and amethod of making the ablative materials. In particular this inventionrelates to carbon ablative materials having a reinforcement componentformed from, as a precursor prior to carbonization, carded and yarn-spunstaple cellulosic fibers. This invention also relates to rocket motorassemblies including the carbon ablative materials.

2. Description of the Related Art

It is generally accepted current industry practice to prepare insulationfor solid propellant rocket motors from a polymeric base compositeimportantly including a carbon cloth. The composite is generallycomposed of the carbon cloth as a woven reinforcement structureimpregnated with a suitable resin matrix. The resin matrix is commonly aphenolic resin, although other resin matrices can be used. For makingthe woven reinforcement structure, current industry practice is toselect a continuous filament non-solvent spun viscose rayon as aprecursor material. The continuous filament viscose rayon, which isespecially formulated for ablative applications, is woven, wound, orotherwise manipulated into its desired configuration and then carbonizedto form a carbon structure exhibiting superior ablation characteristicsand excellent physical properties and processability.

Continuous filament viscose rayon precursor has been established as astandard in the rocket motor industry for making carbon reinforcedstructures of carbon and carbon/carbon ablative materials due to itssuperior ablation characteristics, excellent physical and thermalproperties, and high processability. One of the excellent physicalproperties possessed by composites formed from continuous filamentviscose rayon precursor is a cured composite high warp strength of about144.8 MPa (or about 21,000 lbs/in²) at ambient temperature (about 21° C.or 70° F.), as measured subsequent to carbonization and impregnation ofthe precursor. Warp strength reflects the tolerance of the filament toopposing forces acting along the warp (or longitudinal) filament axis.

However, a major drawback associated with the use of cured compositescomprising wrapped layers of continuous filament viscose rayon, such asfound within the bulk areas of much rocket nozzle insulation, is therelative low across-ply tensile strength possessed by the carbonizedcontinuous filament viscose rayon at operating temperatures experiencedwithin the bulk ablative material (as opposed to the exhaust gassurface) during firing of a rocket motor. Such firing temperatureswithin the bulk ablative material generally can rise to about 400° C.(or 750° F.). Specifically, cured composites comprising wrapped layersof carbonized continuous filament viscose rayon have across-ply tensilestrengths on the order of about 2.07 MPa (or about 300 lbs/in²). Asreferred to herein, across-ply tensile strength is the amount of load,perpendicular to the filament axes, which two overlapping layers offilaments can withstand prior to slippage.

Another significant drawback associated with continuous filament viscoserayon that has recently drawn significant attention involves theavailability of this particular type of continuous filament. Over thepast few years, the only manufacturer producing sufficient quantities ofcontinuous filament viscose rayon to meet industry demands is NorthAmerican Rayon Corp. (NARC) of Elizabethton, Tenn. The capability of theindustry to produce ablative liners and other thermal insulation basedon continuous filament viscose rayon has been jeopardized, however, dueto the cessation of continuous filament viscose fiber production byNARC. There is therefore a need in this industry, previously notsatisfied, to find an effective alternate source or a replacementcandidate for the above-described standard thermal insulation formedfrom continuous filament viscose rayon precursor.

The requirements that a replacement candidate must satisfy in order tobe acceptable and functionally effective are well known to be quitesevere due to the extreme conditions to which the insulation is exposed.These conditions not only include exceedingly high temperatures but alsosevere ablative effects from the hot particles (as well as gases) thattraverse and exit the rocket motor interior, or over the outer surfaceof re-entry vehicle insulators. Unless the insulation will withstandsuch conditions, catastrophic failure may (and has) occurred.

Accordingly, any replacement insulation should exhibit comparabletemperature resistant and ablation characteristics and rheological andphysical properties at least equivalent to those of continuous rayonviscose filament, yet should not otherwise significantly alter themanufacturing process employed for the production of the thermalinsulation. Additionally, due to the large and growing quantities ofsolid propellant rocket motor insulation required by the industry, anysuch replacement reinforcement precursor candidate should be abundantlyavailable now and into the foreseeable future.

An alternative carbon precursor that has been proposed for ablativeapplications is continuous filament polyacrylonitrile (PAN). PANcontinuous filaments disadvantageously possess higher densities thancellulosic materials (1.8 g/cm³ for PAN, compared to 1.48 g/cm³ forcellulosic filaments) and higher thermal conductivities than cellulosicmaterials. Thus, in order to provide a comparable insulation performanceto rayon filaments, rocket motor nozzle insulation or re-entry vehicleinsulation formed from PAN filament must have a greater thickness andweight than a comparable-performing insulation formed from cellulosicmaterials. The replacement material must meet the ablation limits forprotection of the casing (when used as an internal casing insulation)throughout the propellant burn without adding undue weight to the motor.

Accordingly, the search for a functionally satisfactory precursor formaking the reinforcement structure of a composite material requiresdiscovery and implementation of an extraordinarily complex combinationof characteristics. The criticality of the material selection is furtherdemonstrated by the severity and magnitude of the risk of failure. Mostinsulation is of necessity “man-rated” in the sense that a catastrophicfailure can result in the loss of human life—whether the rocket motor isused as a booster for launch of a rocket or is carried tacticallyunderneath the wing of an attack aircraft. The monetary cost of failurein satellite launches is well-publicized and can run into the hundredsof millions of dollars.

Therefore, one of the most difficult tasks in the solid propellantrocket motor industry is the development of a suitable, acceptableinsulation that will meet and pass a large number of test criteria tolead to its acceptability.

Furthermore, any replacement precursor should not be susceptible toobsolescence issues nor discontinuance in future supply thereof.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to address a crucial needin the industry to reformulate the ablative liners and thermal liners ofrocket motors by finding a suitable replacement precursor for makingcarbon-based reinforcement structures. As referred to above, suitablereplacement means a precursor material that can be substituted forcontinuous filament viscose rayon without requiring significant amountsof modification to the impregnating resin composition, component design,and manufacturing process steps and, when carbonized, possess equal orsuperior properties, in particular overall strength, as the thosepossessed by the continuous filament viscose rayon standard.

In accordance with the principles of this invention, these and otherobjects of the invention are attained by the provision of a rocket motorablative material (e.g., an insulation liner or the like) formed from,as a precursor of the carbon reinforcement structure, yarn comprisingcarded and yarn-spun cellulosic (e.g., rayon) fibers. The inventorsdiscovered that staple cellulosic fibers are capable of being processed,such as by spinning, into yarns which, upon patterning (e.g., weaving inany weave style or winding) and subsequent carbonization, can serve as areinforcement of prepregs and can be processed into an insulation linerunder conditions comparable to those of continuous filament viscoserayon.

The inventors also discovered that when staple cellulosic fiberspossessing certain dimensional characteristics are selected, theresulting yarn possesses excellent mechanical strength for rocket motorapplications, yet does not release unacceptable levels of fiberfly—i.e., short, waste fibers—into the air in textile processingoperations such as carding, spinning, and weaving. The former discovery,in particular, was especially surprising because yarns prepared fromcellulosic fibers were expected to possess and do possess significantlylower warp strengths than yarns produced from continuous filamentviscose rayon. However, the inventors found that the lower warpstrengths of the yarns prepared from cellulosic fibers are compensatedfor by the far superior across-ply tensile strength that yarns preparedfrom cellulosic fibers exhibit over continuous filament viscose rayon.

This invention is also directed to a rocket motor assembly comprisingablative materials which comprise reinforcing structures formed from, asa precursor material prior to carbonization, yarn comprising carded andyarn-spun cellulosic fibers. This invention is further directed to aprocess for making a rocket motor assembly comprising the ablativematerials, including nozzle and re-entry vehicle components.

Other objects, aspects and advantages of the invention will be apparentto those skilled in the art upon reading the specification and appendedclaims which, when taken in conjunction with the accompanying drawings,explain the principles of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings serve to elucidate the principles of thisinvention. In such drawings:

FIG. 1 is a schematic cross-sectional view depicting the insulation ofthis invention interposed between a rocket motor casing and solidpropellant;

FIG. 2 is a schematic cross-sectional view identifying some of theregions of a rocket motor assembly in which the insulation of thisinvention may be applied; and

FIG. 3 is a flow diagram of a process for making staple rayon.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the principles of this invention, the replacementprecursor material for preparing carbon reinforcement structures ofrocket motor ablative materials, including nozzle re-entry vehiclecomponents, is yarn comprising carded and yarn-spun cellulosic fibers,especially semi-synthetic cellulosic fibers. As referred to herein andunderstood in the art, carded means fibers subjected to a process orpassed through a machine designed to promote the at least partialseparation and at least partial alignment of fibers. Carding encompassestechniques used in the production of both fine and coarse yarns. Asreferred to herein and understood in the art, yarn-spun means a yarnformed a combination of drawing or drafting and twisting of preparedfibers. Spinning (or yarn-spinning) as referred to herein is notintended to mean techniques consisting of the extrusion of continuousfilaments, which techniques can be performed during solvent-spinning. Asreferred to herein, staple fibers are fibers having lengths suitable foryarn-spinning.

Various semi-synthetic cellulosic fibers can be used in accordance withthis invention. A representative, but non-exhaustive and non-exclusivelist of suitable semi-synthetic cellulosic fibers suitable for use inthe present invention includes standard non-solvent-spun staple rayonfibers, and solvent-spun cellulosic fibers (such as LYOCELL). Cellulosicfibers as referred to herein include those made ofcellulose-derivatives, such as cellulose acetate. As referred to herein,staple fibers are fibers having lengths suitable for spinning.

In the case of non-solvent-spun staple cellulosic fibers, the fiberspreferably have average fiber lengths in a range of from 38 mm to 225mm, such as 100 mm to 150 mm, and, when processed into a yarn, anaverage denier per fiber (dpf) in a range of from 1.5 dpf to 9.0 dpf,such as 5 dpf. The solvent-spun cellulosic fibers preferably havecomparable fiber lengths to those of the staple cellulosic fibers, butan average denier per fiber (dpf) in a range of from 1.1 dpf to 3.0 dpf.One supplier of both types of cellulosic fibers is Acordis of Axis, AL,which supplies, for example, a fiber with a dpf of 5.5 and length of15.24 cm (i.e., 6 inches). The staple standard-rayon cellulosic fibersprovided from this commercial source generally have sodium and zinclevels of 609 ppm and 10 ppm, respectively, which are less than the 1300ppm and 300 ppm of typical continuous filament viscose rayon supplied byNARC. Alternatively, staple standard rayon cellulosic fibers can beprepared by conventional processes well known in the art, such as thatillustrated in FIG. 3. The solvent-spun cellulosic fibers are alsoavailable from Lenzing Fibers of Austria. Solvent-spun cellulosic fibersmade by solvent spinning with N-methylmorpholene-N-oxide are commonlyknown as LYOCELL. The LYOCELL fibers have sodium and zinc levels of 90ppm and 2 ppm, respectively.

The cellulosic fibers are preferably untreated, meaning that they arefree of any distinct metallic, metalloidic, or graphitic coating, atleast prior to (and preferably subsequent to) graphitization.

One of the advantageous features of this invention is that the yarncomprising carded and spun cellulosic fibers may be substituted forconventional continuous filament viscose rayon without significantlyaltering the ablative material manufacturing process. The onlysubstantial alteration in the manufacturing process resides in thedifferences between producing the yarn of this invention and producingconventional continuous filament viscose rayon. Generally, continuousfilament viscose rayon is produced by dissolving cellulose into aviscose spinning solution, and extruding the solution into a coagulatingmedium where the polymer is cellulose and is regenerated as a continuousfilament. On the other hand, the yarn used in the present invention isprepared from staple fibers, which are carded and spun by techniqueswell known in the industry into a tight, compact yarn from the staplefibers. It is understood that other processing techniques may also beused, such as combing and other steps well known and practiced in theart. Preferably, the spinning step is performed by either a worstedprocess or cotton-ring spinning process. The spinning process isadvantageous to keep yarn hairiness to a minimum. By way of example, theyarn may have a weight comparable to the weight of standard yarnspresently used for carbon ablative materials, i.e., about 1650 denier.This may be accomplished with staple fibers by producing a yarn that isapproximately 4.8 English worsted count (Nw), and two-plying the yarn toobtain the 1650 denier configuration. Suitable amounts of twist attainedby spinning can be, for example, 2-12 360° turns per inch, morepreferably 10-12 360° turns per inch.

The yarns are then subject to one or more patterning techniques,including, by way of example, weaving, winding, and plying, into adesired structure. The structure is then carbonized to form thereinforcement of the ablative material. In this regard, the structuringof the yarns into the desired configurations can be performed in thesame manner as that for conventional continuous filament viscose rayon.Carbonization can take lace, by way of example, at a temperature of atleast 1250° C., preferably at east 1350° C. The carbonized reinforcementstructure is then impregnated with an acceptable resin, such as aphenolic resin. A representative phenolic resin is SC1008, availablefrom Borden Chemical of Louisville, Ky.

The inventive ablative and insulation materials can be applied tovarious parts of a rocket assembly, preferably as multi-layeredstructures. For example, the ablative and insulation materials can beused as a chamber internal insulation liner, as shown in FIG. 1.Referring to FIG. 1, the insulation 10, when in a cured state, isdisposed on the interior surface of the rocket motor case 12. Typically,a liner 14 is interposed between the propellant 16 and the insulation10. The insulation 10 and liner 14 serve to protect the case from theextreme conditions produced by the burning propellant 16. Methods forloading a rocket motor case 12 with an insulation 10, liner 14, andpropellant 16 are known to those skilled in the art, and can be readilyadapted within the skill of the art without undue experimentation toincorporate the insulation of this invention. Liner compositions andmethods for applying firers into a rocket motor case are also well knownin the art, as exemplified by U.S. Pat. No. 5,767,221, the completedisclosure of which is incorporated herein by reference.

The ablative and insulation materials can also (or alternatively) beapplied along the flow path through which the combustion products pass,such as shown by the shaded area 20 of the exit nozzle shown in FIG. 2.

The ablative performance and mechanical properties of the carbon clothphenolic prepared from cellulosic fibers as a precursor to thereinforcement are comparable those of carbon cloth phenolics made fromthe aerospace-grade continuous filament viscose rayon in subscale testmotors. For example, although carbonized yarns formed from staplecellulosic fibers exhibit a slightly lower warp strength than yarnsformed from continuous filament viscose rayon (96.5 MPa (that is, 14,000lbs/in²) compared to 144.8 MPa (that is, 21,000 lbs/in²)), carbonizedyarns formed from staple cellulosic fibers have an across-ply strength(5.52 MPa or 800 lbs/in²) twice that of continuous filament viscoserayon (2.76 MPa or 400 Ibs/in²) (at rocket firing temperatures).Although this invention is not currently intended to be limited by anytheory, it is believed that the enhanced across-ply tensile strength ofthe inventive ablative material is attributable to the orientation offibers being offset relative to the yarn axis. As a result, the ends offibers forming the yarn can entangle with the fibers of an adjacentlayer of yarn, thereby increasing the shear strength between the layersof yarn.

This invention will now be described with reference to the followingexamples, which are neither exhaustive nor exclusive of the scope ofthis invention.

EXAMPLES Example 1

3.0 denier per filament (dpf) viscose rayon staple fibers having anaverage length of about 51 mm were spun into yarns using a cotton ringspinning machine. The resulting yarns had a denier of about 825.

Example 2

5.5 dpf viscose rayon staple fibers having an average length of about150 mm were spun into yarns having an average denier of about 825 usinga worsted spinning machine of the type designed to handle long, heavyfibers that can be spun into heavy tows commonly used to make upholsteryfabrics.

Example 3

3.0 dpf LYOCELL staple fibers having an average length of about 51 mmwere spun into yarns having an average denier of about 825 using acotton ring spinning machine.

Example 4

3.0 dpf LYOCELL staple fibers having an average length of about 100 mmwere spun into a yarn having denier of about 825 a using a worsted woolspinning machine.

For each of Examples 1-4, the yarns were spun into heavy tow yarns eachhaving a denier of about 825. This was accomplished by making a Ne(Number English) 6.4 spun yarn. By two-plying (twisting) the yarn into aNe 3.2 yarn, a denier of 1650 was obtained. The resulting yarns werethen woven into fabric in a square woven having a 5 harness satinconfiguration. The fabrics were then carbonized using the standardcarbonization schedules used for ablative carbon fabric filamentcellulosic fibers.

The carbonized fabric was impregnated with a phenolic resin, and inparticular phenol formaldehyde resole resin. The prepreg material was31.0-36.0 wt % resin, 13.0-17.5 wt % carbon black filler, and 46.5 to56.0 wt % carbon fabric.

The following table lists the thermal and mechanical properties ofvarious yarns, carbon cloths, and carbon cloth phenolic ablativematerials tested to compare staple cellulosic precursors with currentfilament rayon precursors.

TABLE 1 Properties of Filament and Staple Cellulosic Fibers in CarbonCloth Phenolic Ablatives Staple Viscose Staple Filament Rayon LYOCELLUnits Rayon (Example 2) (Example 3) Yarn Properties Yarn Denier g/9 KM1650 1650 1650 Yarn Plies Ply 1 2 2 Fibers per Yarn   720 300 550 Denierper Dpf 2.3 5.5 3.0 Filament Woven Fabric Properties Fabric Width cm 152152 152 (inches) (60) (60) (60) Area Weight g/m² 576 576 576 (oz/yd²)(17.0) (17.0) (17.0) Weave Pattern   8 harness 5 harness 5 harness satinsatin satin Carbon Fabric Properties Fabric Width cm 109 109 117(inches) (43) (43) (46) Area Weight g/m² 271 271 271 (oz/yd²⁾ (8.0)(8.0) (8.0) Carbon Content % 97.7 95.7 95.7 Prepreg Properties CarbonContent % 50.6 47.6 45.6 Resin Content % 34.2 36.3 38.4 Filler Content %15.2 16.1 17.0 Cured Composite and Ablative Properties Across Ply MPa26.5 33.6 32.2 Tensile (@ (psi) (3837) (4870) (4665) 21° C. or 70° F.)Across Ply MPa 2.12 4.43 5.21 Tensile (@ (psi) (307) (643) (756) 399° C.or 750° F.) Interlaminar MPa 39.7 50.9 50.1 Shear Strength (psi) (5760)(7385) (7267) Nozzle Erosion μm/s 171 171 173 Rate* (mils/s) (6.74)(6.74) (6.82) Total Heat mm 14.1 12.5 13.1 Effected Depth* (inches)(0.556) (0.494) (0.516) *Based upon solid fuel rocket motor test firingof 35 seconds at 6.2 MPa (900 psi).

As shown in the above Table, the carbon cloth phenolic ablatives formedfrom staple fibers in accordance with this present invention exhibitedmuch higher across ply tensile strengths than the conventionalcontinuous filament viscose rayon precursor at room temperature of 21°C. and operating temperatures of 399° C.

The foregoing detailed description of the preferred embodiments of theinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or exclusive in itsdescription of the precise embodiments disclosed. The embodiments werechosen and described in order to best explain the principles of theinvention and its practical application, thereby enabling others skilledin the art to understand the invention for various embodiments and withvarious modifications covered within the spirit and scope of theappended claims.

1. A method for insulating or thermally protecting a rocket motor assembly comprising a rocket motor casing containing a solid propellant, and a nozzle assembly coupled to the rocket motor casing, said process comprising (a) forming a rocket motor insulation from a prepreg comprising at least one resin matrix impregnated into a carbonized reinforcement, the carbonized reinforcement being formed from, as a precursor prior to carbonization, carded and yarn-spun staple cellulosic fibers and (b) insulating a portion of the rocket motor assembly with the rocket motor ablative material.
 2. The process of claim 1, wherein the staple cellulosic fibers comprise non-solvent-spun rayon fibers.
 3. The process of claim 2, wherein the non-solvent-spun rayon fibers have an average length in a range of from 38 mm to 225 mm, and are spun into yarn having a denier per fiber in a range of from 1.5 dpf to 9.0 dpf.
 4. The process of claim 1, wherein the staple cellulosic fibers are formed from a cellulose-derivative.
 5. The process of claim 1, wherein the staple cellulosic fibers are untreated.
 6. The process of claim 1, wherein said insulating comprises applying the rocket motor ablative material between the solid propellant and the casing surrounding the solid propellant.
 7. The process of claim 1, wherein said insulating comprises applying the ablative material as a bulk ablative material of an exit nozzle liner.
 8. The process of claim 1, wherein said insulating comprises applying the ablative material as a bulk ablative material of a reentry vehicle nose cone.
 9. The process of claim 1, further comprising carbonizing the prepreg at at least 1350° C.
 10. The process of claim 1, wherein the prepreg comprises 31.0-36.0 wt % phenolic resin, 13.0-17.5 wt % carbon black filler, and 46.5 to 56.0 wt % carbon fabric.
 11. Rocket motor insulation comprising a prepreg, said prepreg comprising at least one carbonized reinforcement structure impregnated with at least one resin, said reinforcement structure being formed from, as a precursor prior to carbonization, carded and spun cellulosic fibers.
 12. The rocket motor insulation of claim 11, wherein the staple cellulosic fibers comprise non-solvent-spun rayon fibers.
 13. The rocket motor insulation of claim 12, wherein the non-solvent spin rayon fibers have an average length in a range of from 38 mm to 225 mm, and the yarn has a denier per fiber in a range of from 1.5 dpf to 9.0 dpf.
 14. The rocket motor insulation of claim 11, wherein the staple cellulosic fibers are formed from a cellulosic derivative.
 15. The rocket motor insulation of claim 11, wherein the staple cellulosic fibers are untreated.
 16. A rocket motor assembly comprising the insulation of claim
 11. 17. The rocket motor assembly of claim 16, wherein the insulation is constructed and arranged as a bulk insulation of an exit nozzle liner.
 18. The rocket motor assembly of claim 16, wherein the insulation is constructed and arranged as a bulk insulation of a re-entry vehicle nose cone.
 19. The rocket motor assembly of claim 16, wherein the insulation is interposed between a solid propellant and casing of the rocket motor assembly.
 20. The process of claim 1, wherein said insulating comprises lining the portion of the rocket motor assembly.
 21. A method of thermally protecting a vehicle, the method comprising; forming an ablative material comprising at least one resin matrix impregnated into a carbonized reinforcement including forming the at least one carbonized reinforcement from, as a precursor prior to carbonization, carded and yarn-spun staple cellulosic fibers; and insulating at least a portion of the vehicle including applying the ablative material to the at least a portion of the vehicle.
 22. A method of thermally protecting a surface exposed to combustion products, the method comprising: forming an ablative material comprising at least one resin matrix impregnated into a carbonized reinforcement including forming the at least one carbonized reinforcement from, as a precursor prior to carbonization, carded and yarn-spun staple cellulosic fibers; and insulating at least a portion of a surface along an expected flow path of the combustion products, insulating the at least a portion of the surface including applying the ablative material to the at least a portion of the surface along the expected flow path. 